What Are Stem Cells and How Do They Become Specialized?
Stem cells are unspecialized cells that self-renew by dividing and become specialized types, such as blood, muscle or nerve cells, when signals switch their genes.
Stem cells are cells that can renew themselves by dividing and can also give rise to specialized cell types. A dividing stem cell can make two stem cells, two specialized cells, or one of each, depending on how and where it divides and which signals it receives. Stem cells let an embryo grow into a whole body, and they let adult tissues such as skin and blood keep repairing themselves.
What makes a stem cell different?
Stem cells have two defining properties. The first is self-renewal: they can divide and produce new copies of themselves over a long period. The second is potency, the range of cell types a cell can become. The more types a cell can give rise to, the more potent it is.
Potency falls as an embryo develops. The table summarises the main categories.
| Type | Where it is found | Cells it can form |
|---|---|---|
| Totipotent | Fertilised egg and its earliest divisions | Every cell type, including the placenta |
| Pluripotent | Inner cell mass of an early embryo | Every cell type of the body |
| Multipotent | Adult tissues such as bone marrow | A related set, such as blood cells |
Adult stem cells are usually multipotent. Each kind gives rise only to the cells of its own tissue, so blood-forming cells make blood cells but not nerve cells.
How does a cell become specialized?
Specialization is called differentiation. Nearly every cell in the body carries the same DNA, so the difference between a nerve cell and a liver cell comes from which genes are switched on. The cell reads its DNA through gene expression, and the active genes decide which proteins it makes and what it looks like.
Signals decide which program a cell runs. Molecules released by neighbouring cells bind to receptors on the cell surface, and that turns on specific regulator proteins inside the cell. Those regulators bind to DNA and switch genes on or off. The cell's position in the embryo, and the gradients of signal around it, shape its eventual fate.
Once a cell specializes, chemical marks on its DNA and the proteins that package it help keep that pattern stable. These epigenetic marks are copied when the cell divides, through mitosis. This is why a skin cell's daughters are skin cells and not nerve cells.
Where are stem cells used?
Bone marrow transplants have used blood-forming stem cells for decades, and they are an established treatment for some blood cancers and blood disorders. Other uses are still experimental. Laboratory stem cells are used to study how diseases develop, to test drugs on human cells, and to grow small tissue models for study.
Gene editing tools such as CRISPR-Cas9 are often used on stem cells in the laboratory, so researchers can correct or label specific genes and then watch the resulting cells develop.
What is induced pluripotency?
In 2006 Shinya Yamanaka and colleagues in Japan showed that adult mouse cells could be reprogrammed into pluripotent cells, which became known as induced pluripotent stem cells. The method was extended to human cells in 2007. Yamanaka shared the 2012 Nobel Prize in Physiology or Medicine with John Gurdon for this kind of work.
The reprogrammed cells avoid the need for embryos, but they carry risks of their own. Pluripotent cells that are not controlled can form tumours, so their use in treatment needs careful testing.
Why is stem cell research debated?
Research on embryonic stem cells involves destroying the early embryo from which they are taken, and many people object to this on moral grounds. Others accept it as research on early human life. Laws differ widely between countries, and the debate continues.
Reprogramming adult cells removed the need for embryos, but it raised its own questions about safety and about how such cells should be used. Views differ, and the answers depend on how a society weighs the value of early embryos against the promise of new treatments.
Tags
biology cell division development genetics stem cells
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